Having determined the structure of crystals of deoxy sickle cell hemoglobin (beta 6 glu yields val, one crystal form) at 3 A resolution and deoxy Hb C (beta6 glu yields lys, two crystal forms) at 4.5 and 5.0 A resolution, it is proposed to refine these models so their calculated diffraction patterns best agree with those experimentally observed. We have prepared crystals of deoxy Hb A, S, and C all in the same lattice and space group, and determined the structure of Hb A in this lattice at 3.5 A resolution. Because deoxy Hb C crystals in this lattice are extraordinarily robust and resistant to X-ray damage, it is proposed to extend the analysis to 2.5 A resolution with Hb C. A new orthorhombic crystal of deoxy Hb S has been obtained and its structure has been determined by molecular replacement at 5.5 A resolution. In all the five above crystals and in crystals of deoxy Hb A and deoxy Hb F, published by Perutz's laboratory, there occur strands of molecules that are surrounded by six other strands. The sickle cell mutation site, beta6, frequently participated in the lateral contacts between the molecular strands and it is proposed to examine and catalog these intermolecular contacts.